6/3 NM-B in flex conduit (dry location) run, allowed according to NEC?

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New User
Goal: Replace existing 10-30 (8/3 Al) receptacle with a 14-50 (6/3 Cu).

Ive been reading up and down the interwebs (including NEC) trying to get a concrete answer for this. And based on everything Ive gathered it would be to code to do the following.

First thing I want to say is that I know some will say to just run THHN, but NM-B is much more convenient for me, the cost is the same for the run as THHN, but the conduit is existing, I am upgrading NEMA 10-30 dryer outlet to NEMA 14-50 (to use as a car charger, currently will pull 32A but id like to be capable of pulling the full 14-50 allotted on 50A breaker because who know what this outlet will be used for if I ever sell the house, its just the right thing to do).

So the conduit exists, its a 3/4 inch metal flex conduit (with 8/3 Aluminum feeding the 10-30 outlet, I dont think this is to code anymore? This wire is thick! no way it passes NEC's fill rate, its thicker than the copper 6/3 NM-B, but its probably been there since 1980, was probably to code back then?).

The main reason NM-B is what I want to use is because the run starts in a wall (behind dry wall from the breaker panel), and ends in a wall (behind a drywall where 14-50 is). The FLEX conduit is running along the crossbeams at the top of the garage (exposed beams) from one wall (where breaker panel is) across garage to the other wall (where 14-50 is). Its about 10 feet inside one wall (two 90 degree turns), and another 20 feet or so inside of the outlet wall (one 90 degree turn). Section inside the flex metal conduit is about 30 feet (width of 2 car garage) In order to use THHN id have to rip up the drywall to install conduit inside of both walls, I really dont want to do that.

Everything Ive read here and elsewhere points to NM-B not being allowed inside conduits to being an 'urban legend' and misinterpretations of NEC. But from what Ive read you can not run NM-B in conduit only in the following specific scenario. Wet location, any location outdoors in a conduit is considered wet location). My entire run is inside of a garage, the Flex conduit is at least 8 feet off the ground, not a wet location.

Ive come across people saying to take off the sheath, but thats a no-no because the conductors arent labeled inside, and that will definitely break NEC. And I dont want to do that.

Ive also come across people saying that the over-sizing of the conduit would be impractical, and derating of wires is required. But I thought derating is required only for more than 3 conductors? But even then, its automatically derated being NM-B to 60C even though conductors are actually 90C rated. And at 60C you can pull 50A through 6/3. So it doesnt seem like heat is a concern and derating debate is a moot point since its automatically derated to 50A.

So then fill would be and issue, right? NEC says to treat NM-B as one conductor and take diameter of the entire thing (At thickest point in case its oval, mine is round). I measured it, its 0.5in diameter. I used an online fill table calculator https://www.southwire.com/calculator-conduit with following settings:

Conduit: Flex FMC 3/4
Conductors: User Defined - 0.5 - 1

This comes out to 37% fill. Within NEC spec.

So it seems like I am good to go?

I guess my questions are. Am I not understanding something? Specifically regarding the fill calculation? It seems like a lot of people make the mistake of calculating individual wires inside of NM-B for fill table, but NEC specifically says to treat the whole thing as 1 conductor.

Any advice/help/suggestions are welcome.

Disclaimer: I am an engineer by trade, but not an electrician. Ive done plenty of stuff in college designing low voltage circuits etc. But my knowledge of NEC is purely as a DIYer. So please go easy on me.


Staff member
Thank you for your honesty, and I will go as easy on you as I can:

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